Electrostatics is the branch of science that deals with the
phenomena arising from
what seems to be stationary electric
charges. Since ancient
history it is known that some materials attract light particles
after rubbing.
The greek word
for amber, ήλεκτρον
(electron), gave name
for many areas of natural science. Electrostatic phenomena arise
from the forces that
electric charges carry out on each other. Such forces are described
by Coulomb's
law. Electrostatic phenomena include such as simple as the
attraction of plastic wrap to your hand after you remove it from a
package to apparently spontaneous explosion of grain silos, to
damage of electronic components during manufacturing, to the
operation of photocopiers. Electrostatics involves the buildup of
charge on the surface of
objects due to contact with other surfaces. Although charge
exchange happens whenever any two surfaces contact and
separate, the effects of charge exchange are usually only noticed
when at least one of the surfaces has a high resistance
to electrical flow. This is because the charges that transfer to or
from the highly resistive surface are more or less trapped there
for a long enough time for their effects to be observed. These
charges then remain on the object until they either bleed off to
ground or are quickly neutralized by a discharge: e.g., the familiar
phenomenon of a static 'shock' is caused by the neutralization of
charge built up in the body from contact with nonconductive
surfaces.

The force F imposed by a charge Q on a probe q is
proportional to the
charge of the probe. That is, it can be described by the equation
F=q·E, what defines the electric
field E.

The electrostatic approximation

The validity of the
electrostatic approximation rests on the assumption that the
electric field is irrotational:

\vec\times\vec = 0.

From
Faraday's law, this assumption implies the absence or
near-absence of time-varying magnetic fields:

= 0.

In other words, electrostatics does not require
the absence of magnetic fields or electric currents. Rather, if
magnetic fields or electric currents do exist, they must not change
with time, or in the worst-case, they must change with time only
very slowly. In some problems, both electrostatics and magnetostatics may be
required for accurate predictions, but the coupling between the two
can still be ignored.

Electrostatic potential

Because the electric field is
irrotational, it is possible to express the electric field as the
gradient of a scalar
function, called the electrostatic
potential (also known as the voltage). An electric field, E,
points from regions of high potential, φ, to regions of low
potential, expressed mathematically as

\vec = -\vec\phi.

Fundamental concepts

Coulomb's law

Electric Potential is the amount of work done
per unit charge, in bringing an unit positive charge from infinity
to that point. The fundamental equation of electrostatics is
Coulomb's
law, which describes the force between two point
charges The magnitude of the electrostatic force between two
point electric charges is directly proportional to the product of
the magnitudes of each charge and inversely proportional to the
square of the distance between the charges.Q_1 and Q_2:

The electric field

The electric
field (in units of volts per meter) is defined as the
force (in newtons) per
unit charge (in coulombs). From this definition
and Coulomb's law, it follows that the magnitude of the electric
field E created by a single point charge Q is

Laplace's equation

Triboelectric series

The triboelectric
effect is a type of contact electrification in which certain
materials become electrically charged when coming into contact with
another, different, material, and are then separated. The polarity
and strength of the charges produced differ according to the
materials, surface roughness, temperature, strain, and other
properties. It is therefore not very predictable, and only broad
generalizations can be made. Amber, for example, can acquire an
electric charge by friction with a material like wool. This
property, first recorded by Thales of
Miletus, suggested the word "electricity", from the Greek word
for amber, èlectròn. Other examples of materials that can acquire a
significant charge when rubbed together include glass rubbed with
silk, and hard rubber rubbed with fur.

Electrostatic generators

The presence of surface
charge imbalance means that the objects will exhibit attractive
or repulsive forces. This surface charge imbalance, which yields
static electricity, can be generated by touching two differing
surfaces together and then separating them due to the phenomena of
contact
electrification and the triboelectric
effect. Rubbing two nonconductive objects generates a great
amount of static electricity. This is not just the result of
friction; two nonconductive surfaces can become charged by just
being placed one on top of the other. Since most surfaces have a
rough texture, it takes longer to achieve charging through contact
than through rubbing. Rubbing objects together increases amount of
adhesive contact between the two surfaces. Usually insulators, e.g., substances
that do not conduct electricity, are good at both generating, and
holding, a surface charge. Some examples of these substances are
rubber, plastic, glass, and pith. Conductive
objects only rarely generate charge imbalance except, for example,
when a metal surface is impacted by solid or liquid nonconductors.
The charge that is transferred during contact electrification is
stored on the surface of each object. Static
electric generators, devices which produce very high voltage at
very low current and used for classroom physics demonstrations,
rely on this effect.

Note that the presence of electric
current does not detract from the electrostatic forces nor from
the sparking, from the corona
discharge, or other phenomena. Both phenomena can exist
simultaneously in the same system.

Charge neutralization

Natural electrostatic phenomena are
most familiar as an occasional annoyance in seasons of low
humidity, but can be destructive and harmful in some situations
(e.g. electronics manufacturing). When working in direct contact
with integrated circuit electronics (especially delicate MOSFETs), or in the
presence of flammable gas, care must be taken to avoid accumulating
and suddenly discharging a static charge (see electrostatic
discharge).

Charge induction

Charge induction occurs when a negatively
charged object repels electrons from the surface of a second
object. This creates a region in the second object that is more
positively charged. An attractive force is then exerted between the
objects. For example, when a balloon is rubbed, the balloon will
stick to the wall as an attractive force is exerted by two
oppositely charged surfaces (the surface of the wall gains an
electric charge due to charge induction, as the free electrons at
the surface of the wall are repelled by the negative balloon,
creating a positive wall surface, which is subsequently attracted
to the surface of the balloon). You can explore the effect with a
simulation of the
balloon and static electricity.

'Static' electricity

Before the year 1832, when Michael
Faraday published the results of his experiment on the identity
of electricities, physicists thought "static electricity" was
somehow different from other electrical charges. Michael Faraday
proved that the electricity induced from the magnet, voltaic
electricity produced by a battery, and static electricity are all
the same.

Static electricity is usually caused when certain
materials are rubbed against each other, like wool on plastic or
the soles of shoes on carpet. The process causes electrons to be
pulled from the surface of one material and relocated on the
surface of the other material.

A static shock occurs when the surface of the
second material, negatively charged with electrons, touches a
positively-charged conductor. Or Vice-Versa.

Static electricity is commonly used in xerography, air filters,
and some automotive paints. Static electricity is a build up of
electric charges on two objects that have become separated from
each other. Small electrical components can easily be damaged by
static electricity. Component manufactures use a number of antistatic
devices to avoid this.

Static electricity and chemical industry

When different materials are brought together and
then separated, an accumulation of electric charge can occur which
leaves one material positively charged while the other becomes
negatively charged. The mild shock that you receive when touching a
grounded object after walking on carpet is an example of excess
electrical charge accumulating in your body from frictional
charging between your shoes and the carpet. The resulting charge
build-up within your body can generate a strong electrical
discharge. Although experimenting with static electricity may be
fun, similar sparks create severe hazards in those industries
dealing with flammable substances, where a small electrical spark
may ignite explosive mixtures with devastating consequences.

A similar charging mechanism can occur within low
conductivity fluids flowing through pipelines - a process called
flow electrification. Fluids which have low electrical conductivity
(below 50 pico siemens/cm, where pico siemens/cm is a measure of
electrical conductivity), are called accumulators. Fluids having
conductivities above 50 pico siemens/cm are called
non-accumulators. In non-accumulators, charges recombine as fast as
they are separated and hence electrostatic charge generation is not
significant. In the petrochemical industry, 50 pico siemens/cm is
the recommended minimum value of electrical conductivity for
adequate removal of charge from a fluid.

An important concept for insulating fluids is the
static relaxation time. This is similar to the time constant (tau)
within an RC circuit.
For insulating materials, it is the ratio of the static dielectric
constant divided by the electrical conductivity of the
material. For hydrocarbon fluids, this is sometimes approximated by
dividing the number 18 by the electrical conductivity of the fluid.
Thus a fluid that has an electrical conductivity of 1 pico siemens
/cm will have an estimated relaxation time of about 18 seconds. The
excess charge within a fluid will be almost completely dissipated
after 4 to 5 times the relaxation time, or 90 seconds for the fluid
in the above example.

Charge generation increases at higher fluid
velocities and larger pipe diameters, becoming quite significant in
pipes or larger. Static charge generation in these systems is best
controlled by limiting fluid velocity. The British standard BS PD
CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for
Control of Undesirable Static Electricity prescribes velocity
limits. Because of its large impact on dielectric constant, the
recommended velocity for hydrocarbon fluids containing water should
be limited to 1 m/s.

Bonding and earthing are the usual ways by which
charge buildup can be prevented. For fluids with electrical
conductivity below 10 pico siemens/cm, bonding and earthing are not
adequate for charge dissipation, and anti-static additives may be
required.

Applicable Standards

1.BS PD CLC/TR 50404:2003 Code of Practice for
Control of Undesirable Static Electricity